In screw compressors, unloading is typically achieved by means of a slide valve which is reciprocated along an axis which is parallel to the axes of the helical rotors. The position of the slide valve relative to the rotors adjusts the size and duration of opening of the suction port by changing the effective length of the rotors and thereby the trapped volume. Because the slide valve position is thus directly related to the compressor output, the position of the slide valve has been sensed and a feedback signal provided to the control system which controls compressor capacity by positioning the slide valve. Additionally, an indicator can be positioned by the slide valve as it is moved.
In the past, the feedback and/or indicator structure ordinarily required the passage of a rod through the compressor casing. The rod would move with the slide valve and, although the rod is sealed, there have been problems due to leakage. The sealing problems were overcome in U.S. Pat. No. 3,738,116 which has the rod located in a sealed tube. The rod is in an inner tube and carries a magnetic member which coacts with an annular magnetic member surrounding the inner tube. The position of the annular magnetic member gives a visual indication of the slide valve position. Additionally, the annular magnetic member coacts with reed switches located in the inner wall of the outer tube when the annular magnetic member and therefore the slide valve is at one or more specific locations. This, however, did not satisfy the need for a direct, continuous output indicative of the slide valve position which was provided by commonly assigned U.S. Pat. No. 4,743,170 which is hereby incorporated by reference. In that patent a stainless steel tube is used to provide pressure and media isolation. Permanent magnets are used to transfer mechanical energy across the barrier provided by the stainless steel tube and the resultant permanent magnet "coupling" mechanism permits the movement of a position indicating device such as a potentiometer with movement of the slide valve.
U.S. Pat. No. 4,610,612 discloses a rotary screw compressor using linearly variable differential transformers (LVDT) coupled to the slide valves. The rods connecting with the LVDT go through the compressor housing and require sealing. An LVDT has three coils which form the transformer. The primary coil is used to provide excitation to the magnet assembly and ultimately to the two secondary windings. The transfer of the excitation forms the basis for the position measurement.
In an LVDT, a sinusoidal source excites the magnet assembly and is of a high frequency, typically 10-20 kHz. The frequency is high enough to reduce the permeability of and the subsequent size of the magnetic core. The construction of the magnetic core must be precisely controlled in order to provide equal magnetic properties over the entire length of the magnet. An anomaly in the magnet will translate into a corresponding anomaly in the output of the LVDT.
The magnetic excitation of the core is transferred to the secondary windings. The construction of the secondary windings is also very critical. Each winding must contain a precise number of turns and be matched to the other secondary winding if the LVDT is to function properly. When the magnetic core is centered between the secondary coils, the output of the respective secondary coils will be equal and the differential output from the LVDT will be zero. Moving the magnetic core off mechanical center will result in a voltage increase in one of the coils and a subsequent decrease in the voltage output from the opposite coil. The voltage differential between the two coils forms the output from the LVDT.
The LVDT must be designed and sized to operate for a specific length of travel which is, typically, less than that of either of the secondary coils. The LVDT is, typically, twice as long as the length of travel it is intended to measure.